Percutaneous transluminal coronary angioplasty (PTCA) is a common procedure for treating heart disease. A problem associated with the PTCA includes the formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining, and to reduce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain the vascular patency.
Stents are used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the desired site. Local delivery produces fewer side effects and achieves more favorable results.
However, the use of drug eluting stents (DESs) has resulted in a new problem, late stent thrombosis, the forming of blood clots long after the stent is in place. It was deduced that the formation of blood clots was most likely due to delayed healing which was postulated to be a side-effect of the use of cytostatic drugs.
Another potential shortcoming of the foregoing method of medicating stents is the control of the release rate of a therapeutic agent. The active agent can be released from a stent by either diffusion, or swelling followed by diffusion and degradation or erosion. The hydrophobicity of the polymer is critical in dissolving the therapeutic agent. The commonly used therapeutic agents have limited or low solubility posing a serious obstacle for the drug's release kinetics from a stent. Thus, there is a need to maximize the solubility of a therapeutic agent in the polymer and to optimize the release rate of a therapeutic agent.
To address the above situation, stents can be fabricated from materials that are biocompatible, biodegradable and, if desired, bio-absorbable. The goal is for the stent to have a biocompatible coating which demonstrates great safety with regard to stent thrombosis. Ideally, the stent coatings should preferably lower acute and sub-acute thrombosis rates. The coating material selected must not only have sufficient mechanical properties but also show excellent coating integrity. The preceding problem has been at least partially ameliorated by the use of increasingly biocompatible materials and/or biocompatible coating.
What is needed is an implantable medical device that includes a polymer coating which maximizes the solubility of a therapeutic agent in the polymer and optimizes the release rate of a therapeutic agent. While this would be particularly useful with regard to coronary stents, it would also provide substantial benefit to any manner of implantable medical devices. Such implantable medical devices for use as drug delivery systems should also demonstrate excellent mechanical properties when implanted in a patient. The present invention provides such implantable medical devices.